Turbochargers are a type of forced induction system. They deliver air, at greater density than would be possible in the normally aspirated configuration, to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. This can enable the use of a smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, thus reducing the mass and aerodynamic frontal area of the vehicle.
The wheels in turbochargers are designed to operate at nearly maximum stress at the peak of the operating regime. This produces the best compromise between stress and mass, with respect to the design of the wheels, to provide the lowest inertia at the desired efficiency and pressure ratio for the wheels. The inference of this compromise is that the wheels of a turbocharger cannot be run in an overspeed condition or the wheels will be overstressed. Along with overstress conditions, comes the damage accumulation due to the speed cycles the wheels undergo. Damage accumulation is a major factor in low cycle fatigue (LCF) in turbocharger wheels.
The rotating assembly of a turbocharger rotates at exceptionally high speed in the order of 200,000 RPM for a small rotor and 80,000 RPM for a large rotor. Since the invention of the turbocharger, rotational speeds have climbed steadily. Transient response is a time based metric of the speed change for an accelerating or decelerating engine. The function of the turbocharger is a strong factor in engine transient response. A typical transient response measurement protocol is the time it takes for the engine to get from high idle engine speed to 80% of maximum torque. Because the turbocharger rotational speed is an important component in turbocharger transient response, the faster the speed of the turbocharger at engine high idle speed the shorter the time it takes to get to maximum engine torque. While this is a simplistic view, along with high turbocharger speed at engine high idle, comes high turbocharger speed at maximum engine torque; so the turbocharger must be protected from overspeed.
Before the advent of electronic engine controls, the selection and design of compressor wheels was rather simple with large margins of safety and large altitude compensation margins, so, in general, the wheels ran at comparatively sedate speed levels. With the introduction of electronic engine controls, it became possible to run the turbocharger at, or near, the design limit all the time so accumulation of damage in the wheel became acute, and LCF failures became more common.
Also introduced commercially were variable geometry turbochargers and regulated two stage turbochargers. In each of these cases, it became possible for a turbocharger to “overspeed”. With a VTG, closing the vanes down accelerates the exhaust gas onto the turbine wheel blades causing the speed of the rotating assembly to increase. The rotating assembly is associated with inertia, and as a result there is a lag time between the closing of the vanes from high speed and the stop in acceleration of the rotating assembly, and this can cause a maximum speed overshoot. Further, with a regulated two stage turbo, the smaller stage is used for fast engine acceleration and the larger stage is used for supplying sufficient mass flow at the high end of the engine operating regime. If the changeover from small turbocharger to larger turbocharger is delayed, then the speeds of the small turbocharger can go out of range, and the turbo can overspeed.
In order to exercise control over the speed of the rotating assembly, turbochargers are sometimes equipped with speed sensors. Speed sensors come in several types. Variable reluctance (VR) sensors use a coil around a magnet in the end of the sensor. As the rotating target cyclically gets closer and further from the magnet, the attraction forces change the shape of the magnetic field, which induces a measurable voltage in the coil. Another type is an electromagnetic sensor which reads the cyclic impedance of a flat on a rotating shaft. As depicted in FIGS. 1 and 2, the “shaft” (11) of the shaft and wheel which supports the journal bearings has a flat (13) fabricated into it approximately between the journal bearings (64). For symmetrical/balance/stress reasons, sometimes there are opposing flats fabricated into the shaft. In the above sensors types, the flat on the rotating shaft passes by the end (73) of the sensor, thus providing a signal as the distance from the end of the sensor to the proximate surface of the shaft (11) cyclically alters due to the difference in radius from the center of the shaft to the diameter of the shaft and then the flat (13) on the shaft. The signal emanating from the sensor (70) is transmitted to the engine electronic control system via a cable (75)
To fit the speed sensor (70) to a turbocharger bearing housing (20), the threaded portion (71) of the sensor (70) is threaded into a complementary threaded part (21) of the bore into which the speed sensor is located. The depth of the sensor is set and maintained by an inwards facing surface (76) on the sensor locating against an outwards facing abutment on the bearing housing (20). The “gap” between the inner end (73) of the sensor and the outer surface of the rotating shaft (11) must be both set and held constant for the sensor to operate consistently and accurately.
The end of the sensor (73), which is in the realm of 0.75 mm in diameter, generally must be within less than 1 mm proximity with the shaft surface. For designs in which the journal bearings (64) of the turbocharger are axially separated by a spacer (67), the spacer is typically fabricated from a ferrous metal which would shield the rotating shaft's flat-to-diameter cyclic distance variation. Accordingly, a window (68) is provided in the spacer, and the speed sensor tip protrudes through the window. The protrusion of the shaft (72) of the sensor (70) into the spacer (67) constrains the spacer from rotating about the turbocharger axis (1). In the assembly process the spacer is held in position (such that the window (68) in the spacer is aligned with the axis of the sensor) by a magnetic tool inserted into the journal bearing bore before the compressor-end journal bearing is assembled into the journal bearing bore.
Because such sensors are delicate electronic articles living in a thermally and vibrationally harsh environment, they are prone to failure. To replace the speed sensor (70), a technician must unscrew the sensor from the bearing housing (20) and replace the sensor by inserting the end (73) of the sensor (70) through the bearing housing and then spacer window (68). During the removal of the sensor, it is quite easy for the rotating assembly to move rotationally and drag the journal bearing spacer with said rotation, thus moving the window (68) in the spacer (67) from its prior alignment with the axis of the sensor. Since the spacer is at the bottom of a long bore, the technician can not see the position of the window in the spacer relative to the bore into which the sensor is fitted. Failure for the end of the sensor to pass through the window (68) in the spacer can result in damage to the sensor and potential damage to the spacer and bearings.
It is known from WO2012024092, assigned to the assignee of the present application, to fix a rotational speed sensor in a bearing housing recess, and to arrange a resilient sleeve in the bearing housing recess around the rotational speed sensor and engaging with one end into the sensor recess. However, once assembled, it is difficult to gain access to and remove the resilient sleeve for turbocharger overhaul.
Further, the fluctuating torque transmitted from the shaft to the bearing spacer during normal turbocharger operation causes the spacer to wearing and damage the outside of the sensor probe. There is a need to prevent such damage to the sensor probe.